1. Field of the Invention
The present invention relates to the construction of an electronic device. The present invention particularly relates to an active matrix type electronic device having a thin film transistor (TFT) manufactured on an insulating body and a driving method of the active matrix type electronic device.
2. Description of the Related Art
In recent years, an EL (electroluminescence) display has been attracting attentions as a flat panel display that is to replace an LCD (liquid crystal display), and is actively researched.
There are generally two types of driving systems for the LCD. One type is a passive matrix type used in an STN-LCD, etc. The other type is an active matrix type used in a TFT-LCD, etc. Similarly, there are generally two kinds of driving systems in the EL display. One type is a passive matrix type, and the other type is an active matrix type.
In the case of the passive matrix type, wiring to serve as an electrode is arranged in each of upper and lower portions of an EL element. A voltage is sequentially applied to the wirings, and an electric current flows through the EL element so that the EL element is lighted. On the other hand, in the case of the active matrix type, each pixel has a TFT, and a signal can be held within each pixel.
FIG. 21 shows an example of the structure of an active matrix type electronic device by digital driving. A pixel portion 2101 is arranged in the center. In the periphery of the pixel portion 2101, a source signal line side driver circuit 2102 is arranged to control source signal lines, and a gate signal line side driver circuit 2106 is arranged to control gate signal lines. In FIG. 21, the gate signal line side driver circuit 2106 is provided only on one side of the pixel portion 2101. However, considering operation efficiency and reliability in actual driving, it is desirable to arrange the driver circuits on both sides so as to sandwich the pixel portion 2101. Further, a power supply portion for supplying electric current to EL elements is connected to respective current supply lines of the pixel portion 2101.
The EL element is comprised of an anode, a cathode, and a layer containing an organic compound that provides electro luminescence (luminescence generated by applying electric field) (the layer hereinafter referred to as an EL layer). The luminescence from an organic compound can be divided into light emission upon returning from singlet excitation to the base state (fluorescence) and light emission upon returning from triplet excitation to the base state (phosphorescence). Both kinds of light emission can be used in electronic devices to which the present invention is applicable.
The EL layer defined herein includes all the layers that are provided between an anode and a cathode through this specification. Specifically, the EL layer is comprised of a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer, and some other layers. The basic structure of an El element is a laminate in which an anode, a light emitting layer and a cathode are sequentially layered. Other types of EL layer structure are a laminate in which an anode, a hole injecting layer, a light emitting layer and a cathode are sequentially layered, and a laminate in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer and a cathode are sequentially layered.
In this specification, the EL element refers to an element composed of an anode, an EL layer and a cathode.
The source signal line side driver circuit 2102 has a shift register 2103, a first latch circuit 2104 and a second latch circuit 2105. The shift register 2103 receives input of a source side clock signal (S-CLK) and a source side start pulse (S-SP). The first latch circuit 2104 receives input of a digital image signal (digital data). The second latch circuit 2105 receives input of a latch pulse.
The gate signal line side driver circuit 2106 has a shift register (not shown). The shift register receives input of a gate side clock signal (G-CLK) and a gate side start pulse (G-SP).
Drive of the circuits will be described. Reference symbols in FIG. 21 will be used in the description.
In the source signal line side driver circuit 2102, a source side clock signal (S-CLK) and a source side start pulse (S-SP) are inputted to the shift register 2103. The shift register 2103 outputs pulses successively in response to these inputted signals. The pulses successively outputted from the shift register are inputted through a buffer and other circuits (not shown) to the first latch circuit 2104, so that digital image signals (digital data) are successively held (latched) at the respective stages. Upon completion of storing the data at the last stage of the first latch circuit 2104, latch pulses are inputted to the second latch circuit 2105. Then the data that have been held in the first latch circuit 2104 are transferred all at once to the second latch circuit 2105 through a buffer and other circuits (not shown).
In the gate signal line side driver circuit 2106, a gate side clock signal (G-CLK) and a gate side start pulse (G-SP) are inputted to the shift register (not shown). The shift register outputs pulses successively in response to these inputted signals. The pulses pass through a buffer and other circuits (not shown) and are successively outputted as gate signal line selecting pulses to sequentially select gate signal lines.
The data transferred to the second latch circuit 2105 of the source signal line side driver circuit 2102 are written in pixels on the row selected by the gate signal line selecting pulse. This operation is repeated to display an image.
A description given next is on drive of the pixel portion. FIGS. 22A and 22B show a part of the pixel portion 2101 of FIG. 21. FIG. 22A shows a matrix consisting of 3×3 pixels. A section defined by a dotted line frame 2200 is one pixel and an enlarged view thereof is shown in FIG. 22B. In FIG. 22B, reference symbol 2201 denotes a TFT functioning as a switching element when a signal is written in a pixel (hereinafter referred to as switching TFT). The switching TFT 2201 may have the polarity of either an n-channel TFT or a p-channel TFT. Reference symbol 2202 denotes a TFT functioning as an element for controlling a current supplied to an EL element 2203 (current controlling element) (the TFT will be called an EL driver TFT). If a p-channel TFT is used for the EL driver TFT 2202, the TFT is placed between an anode 2209 of the EL element 2203 and a current supply line 2207. If an n-channel TFT is used for the EL driver TFT 2202, on the other hand, the TFT is placed between a cathode 2210 of the EL element 2203 and a cathode electrode 2208. However, because of preference to source grounding in light of TFT behavior and limitations in manufacture of the EL element 2203, a general and frequently employed method is to use a p-channel TFT for the EL driver TFT 2202 and place the EL driver TFT 2202 between the anode 2209 of the EL element 2203 and the current supply line 2207. Reference symbol 2204 denotes a storage capacitor for storing a signal (voltage) inputted from a source signal line 2206. In FIG. 22B, one of terminals of the storage capacitor 2204 is connected to the current supply line 2207. However, it may be connected to an exclusive wiring. The switching TFT 2201 has a gate electrode connected to a gate signal line 2205, and has a source region connected to the source signal line 2206.
An operation of the circuit of the active matrix type electronic device will next be explained with reference to FIGS. 22A and 22B. First, when the gate signal line 2205 is selected, a voltage is applied to a gate electrode of the switching TFT 2201, and the switching TFT 2201 attains a turned-ON state. Then, a signal (voltage) of the source signal line 2206 is accumulated in the storage capacitor 2204. The voltage of the storage capacitor 1504 becomes a voltage VGS between the gate and the source of the EL driver TFT 2202 so that an electric current according to the voltage of the storage capacitor 2204 flows through the EL driver TFT 2202 and the EL element 2203. As a result, the EL element 2203 is lighted.
Luminance of the EL element 2203, i.e., an electric current amount flowing through the EL element 2203 can be controlled by VGS of the EL driver TFT 2202. VGS is the voltage of the storage capacitor 2204, and is a signal (voltage) inputted to the source signal line 2206. Namely, the luminance of the EL element. 2203 is controlled by controlling the signal (voltage) inputted to the source signal line 2206. Finally, the gate signal line 2205 is set to a non-selected state, and the gate of the switching TFT 2201 is closed, and the switching TFT 2201 is set to a turned-OFF state. At that time, electric charges accumulated in the storage capacitor 2204 are held. Accordingly, VGS of the EL driver TFT 2202 is held as it is, and an electric current according to VGS continuously flows through the EL driver TFT 2202 and the EL element 2203.
Driving of an EL element etc. are reported in SID99 Digest: P372: “Current Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFT”, ASIA DISPLAY 98: P217: “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, Euro Display99 Late News: P27: “3.8 Green OLED with Low Temperature Poly-Si TFT”, etc.
EL displays of late are required to have higher definition as well as to be equipped with a larger screen. However, enhancing the definition of the pixel portion by reducing the pixel pitch raises a problem of insufficient space for placing a driver circuit. For instance, if the definition is to be improved from VGA to XGA without changing the size of the panel, the pixels in the horizontal direction are increased in number from 640 pixels to 1024 pixels. The width of one pixel in this case is reduced to 62.5%, and hence the width for placing one stage of a source signal line side driver circuit is also reduced to 62.5%.
In order to solve the problem above, the driver circuit has to be further reduced in size. However, this is not so easy a solution to carry out when taking into consideration rules in designing, reliability in circuit behavior, yield, etc.